The present invention relates to distillation trays for use in distillation columns.
A conventional distillation column includes a plurality of horizontal distillation trays which are spaced apart vertically. Each tray has a plurality of evenly spaced vapour passages therethrough and is associated with downcomers. Each downcomer has an inlet which receives liquid from a tray above and an outlet from which the liquid discharges onto the adjacent lower tray. When a liquid mixture having two or more components is distilled, liquid descends the column tray by tray and, as it flows across the trays, it comes into intimate contact with vapour rising through the passages. Typically the column is provided with a condenser at its top to provide the downward passage of liquid and a boiler to promote the upward flow of vapour. Thus, as the vapour ascends the column it becomes progressively colder and as the liquid descends the column it becomes progressively warmer. The aforesaid intimate contact between the ascending vapour and the descending liquid causes the vapour to give up to the liquid more and more of the less volatile component or components of the mixture being distilled. Thus, the vapour as it ascends the column becomes progressively richer in the more volatile component or components of the mixture.
As a result, the mixture separates into two fractions of different composition, one liquid and the other vapour.
A conventional distillation tray is circular or part-circular in shape. In use, it may receive liquid along one chord and discharge liquid over an opposite chord. The flow path from liquid inlet to liquid outlet around the periphery of the tray is longer than that defined by a straight line extending perpendicularly from the chordal liquid inlet to the chordal liquid outlet.
The liquid therefore tends to dwell at the periphery of the tray. As a result, a greater degree of contact takes place between the relatively cold liquid and the relatively warm vapour at the periphery of the tray than, say, at its centre. The liquid at the periphery of the tray becomes more depleted in its more volatile component or components than the liquid at the centre. This liquid is then less able to transfer the more volatile component or components to the vapour with which it comes into contact. The composition of the vapour ascending the column through peripheral tray areas therefore progressively diverges from that of the vapour ascending the column through more central areas, the former vapour being richer in the less volatile component or components of the mixture than the latter vapour. The peripheral areas of the trays are therefore less effective in fractionating the mixture than the more central areas, and accordingly more trays are employed in the distillation column than would be needed were the peripheral and central areas to be equally effective.
Various means have been proposed to ameliorate this problem. For example, rather than relying on a hydraulic or liquid gradient to cause flow of liquid across a tray from its inlet to its outlet, U.S. Pat. No. 3,417,975 proposes employing in addition to the conventional uniform pattern of fixed size openings or perforations with walls normal to the tray surface, a uniform pattern of obliquely inclined openings, the latter being oriented in the downstream direction of liquid flow. Such slotted sieve trays, as they have been called, are said to perform with improved efficiency owing to the elimination of longitudinal hydraulic gradient in the liquid on the tray. The neutralisation of the hydraulic gradient is stated to produce uniform resistance to vapour penetration through the liquid on the tray, and both vapour and liquid flows are distributed more evenly over the tray. In order to provide improved uniformity on the tray U.S. Pat. No. 3,759,498 discloses employing bubble promoters near the liquid inlet to the tray with a view to preventing areas of inactivity occuring in the vicinity of the inlet. This has the effect of tending to overburden the tray with liquid in the vicinity of the liquid outlet. U.S. Pat. No. 3,759,498 teaches employing a nonuniform distribution of the oblique slots (though it still employs the conventional uniform distribution of the conventional perforations) with there being a greater slot density in that half of the tray nearer the liquid outlet than in that half nearer the liquid inlet so as to counteract this problem. As a further measure specifically intended to enhance the liquid flow at the periphery of the tray, those slots in the outlet half of the tray towards its periphery are pointed in the direction of the liquid outlet so that the thrust imparted by the vapour to the liquid is in the direction of the outlet and thus the liquid at the periphery of the tray is urged towards the outlet and is prevented from dwelling too long at the periphery. U.S. Pat. No. 3,759,498 further teaches trying to equalise over the whole active surface of the tray the product of the liquid or froth height and the volumetric fraction of liquid in the froth.
We believe that the approach of using oblique slots to improve tray efficiency, as is for example described in the aforementioned U.S. patent specifications, can achieve only partical success.